BCS everywhere else: from Atoms and Nuclei to the Cosmos. Gordon Baym University of Illinois

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1 BCS everywhere else: from Atoms and Nuclei to the Cosmos Gordon Baym University of Illinois October 13, 2007

2 Wide applications of BCS beyond laboratory superconductors Pairing of nucleons in nuclei Neutron stars: pairing in neutron star matter Pairing of quarks in degenerate quark-gluon plasmas Elementary particle physics broken symmetry Cold fermionic atoms Helium-3

3 BCS applied to nuclear systems Pairing of even numbers of neutrons or protons outside closed shells *David Pines brings BCS to Niels Bohr s Institute in Copenhagen, Summer 1957, as BCS was being finished in Urbana. *Aage Bohr, Ben Mottelson and Pines (57) suggest BCS pairing in nuclei to explain energy gap in single particle spectrum odd-even mass differences *Pairing gaps deduced from odd-even mass differences: Δ ' 12 A -1/2 MeV for both protons and neutrons

4 B. Mottelson, M. Goeppert-Mayer, H. Jensen, Aa. Bohr Conference on Nuclear Structure, Weizmann Institute, Sept. 8-14, 1957

5 Energies of first excited states: even-even (BCS paired) vs. odd A (unpaired) nuclei Energy gap

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7 Rotational spectra of nuclei: E = J 2 / 2I, indicate moment of inertia, I, reduced from rigid body value, I cl.. Reduction of moment of inertia due to BCS pairing = analog of Meissner effect. Detailed calculations by Migdal (1959).

8 BCS pairing of nucleons in neutron stars Mass ~ 1.4 M sun Radius ~ km Temperature ~ K Surface gravity ~10 14 that of Earth Surface binding ~ 1/10 mc 2 Mountains < 1 mm 1 S 0 neutrons 1 S 0 protons 3 P 2 neutrons Density ~ 2x10 14 g/cm 3

9 Neutron drip Beyond density ρ drip g/cm 3 neutron bound states in nuclei become filled through capture of high Fermi momentum electrons by protons: e - +p n +ν. Further neutrons must go into continuum states. Form degenerate neutron Fermi sea. Neutrons in neutron sea are in equilibrium with those inside nucleus Protons never drip, but remain in bound states until nuclei merge into interior liquid.

10 Superfluidity of nuclear matter in neutrons stars Migdal 1959, Ginzburg & Kirshnits 1964; Ruderman 1967; GB, Pines & Pethick,, 1969 First estimates of pairing gaps based on scattering phase shifts CRUST LIQUID CORE Neutron fluid in crust BCS-paired in relative 1 S 0 states Neutron fluid in core 3 P 2 paired Proton fluid 1 S 0 paired n=hoffberg et al. 1970, p=chao et al. 1972

11 Quantum Monte Carlo (AFDMC) 1 S 0 nn gap in crust: Fabrocini et al, PRL 95, (2005) BCS for different interactions QMC (black points) close to standard BCS (upper curves) Green s function Monte Carlo (Gezerlis 2007)

12 Rotating superfluid neutrons Rotating superfluid threaded by triangular lattice of vortices parallel to stellar rotation axis Quantized circulation of superfluid velocity about vortex: Bose-condensed 87 Rb atoms Schweikhard et al., PRL (2004) Vortex core 10 fm Vortex separation 0.01P(s) 1/2 cm; Vela contains vortices Angular momentum of vortex =N~(1-r 2 /R 2 ) decreases as vortex moves outwards => to spin down must move vortices outwards Superfluid spindown controlled by rate at which vortices can move against barriers, under dissipation

13 Superconducting protons in magnetic field Even though superconductors expel magnetic flux, for magnetic field below critical value, flux diffusion times in neutron stars are >> age of universe. Proton superconductivity forms with field present. Proton fluid threaded by triangular (Abrikosov) lattice of vortices parallel to magnetic field (for Type II superconductor) Quantized magnetic flux per vortex: = φ 0 = G. Vortex core 10 fm, n vort = B/φ 0 => spacing ~ 5 x cm (B /10 12 G) -1/2

14 Pulsar glitches Sudden speedups in rotation period, relaxing back in days to years, with no significant change in pulsed electromagnetic emission 90 glitches detected in 30 pulsars Vela (PSR ) Period=1/Ω=0.089sec 15 glitches since discovery in 1969 ΔΩ/Ω ~ 10-6 Largest = on Jan. 16, 2000 Moment of inertia gcm 2 => ΔE rot ~ erg Feb. 28, 1969 Reichley and Downs, Nature 1969 Radhakrishnan and Manchester, Nature 1969 Crab (PSR ) P = 0.033sec 14 glitches since 1969 ΔΩ/Ω 10-9

15 Pairing in high energy nuclear/particle physics Vacuum condensates: quark-antiquark pairing underlies chiral SU(3) SU(3) breaking of vacuum=> Experimental Bose-Einstein decondensation Karsch & Laermann, hep-lat/ Broken symmetry -- Particle masses via Higgs field L m = ghψ γ 0 ψ => g hhi ψ γ 0 ψ => m = ghhi h BCS pairing of degenerate quark matter color superconductivity

16 Color pairing in quark matter Review: Alford, Rajagopal, Schaefer & Schmitt, arxiv: Temperature 150 MeV Ultrarelativistic heavy-ion collisions Hadronic matter Nuclear liquid-gas Quark-gluon plasma 1 GeV Neutron stars Baryon chemical potential? 2SC CFL Superfluidity condensate of paired quarks => superfluid baryon density (n s ) Color Meissner effects transverse color fields screened on spatial scale ~ London penetration depth ~ (μ/g 2 n s ) 1/2 Two interesting phases: 2SC (u,d) Color-flavor locked (CFL) (m u =m d =m s )

17 BCS paired fermions: a new superfluid High T: Boltzmann distribution Observing Statistics 7 Li vs. 6 Li Bosons: BEC Degenerate fermions in two hyperfine states BCS pairing Low T: Degenerate gas Hulet Produce trapped degenerate Fermi gases: 6 Li, 40 K Increase attractive interaction with Feshbach resonance At resonance have unitary regime : no length scale resonance superfluidity Experiments: JILA, MIT, Duke, Innsbruck,...

18 Controlling the interparticle interaction Effective interparticle interaction short range s-wave: V(r 1 -r 2 ) = (4π~ 2 a/m) δ (r 1 -r 2 ); a= s-wave atom-atom scattering length weakly bound molecule in closed channel Li Scattering Length ( a O ) Magnetic Field ( G ) Broad resonance around 830 Gauss Increasing magnetic field through resonance changes interactions from repulsive to attractive; very strong in neighborhood of resonance

19 Feshbach resonance in atom-atom scattering open channel closed channel open channel s-wave magnetic moment: μ μ + Δ μ Scattering amplitude M 2 E c E o E c -E 0 Δμ B +... Low energy scattering dominated by bound state closest to threshold Adjusting magnetic field, B, causes level crossing and resonance, seen as divergence of s-wave scattering length, a:

20 BEC-BCS crossover in Fermi systems Continuously transform from molecules to Cooper pairs: D.M. Eagles (1969) A.J. Leggett, J. Phys. (Paris) C7, 19 (1980) P. Nozières and S. Schmitt-Rink, J. Low Temp Phys. 59, 195 (1985) Pairs shrink T c /T f 0.2 T c /T f e -1/k fa 6 Li

21 Relation of Bose-Einstein condensation and BCS pairing? The two phenomena developed along quite different paths Our pairs are not localized..., and our transition is not analogous to a Bose-Einstein condensation. BCS paper Oct "We believe that there is no relation between actual superconductors and the superconducting properties of a perfect Bose-Einstein gas. The key point in our theory is that the virtual pairs all have the same net momentum. The reason is not Bose-Einstein statistics, but comes from the exclusion principle...." Bardeen to Dyson, 23 July 1957

22 Phase diagram of cold fermions vs. interaction strength Temperature Free fermions +di-fermion molecules T a>0 c /E F 0.23 T c BEC of di-fermion molecules 0 Free fermions a<0 T c E F e -π/2k F a (magnetic field B) BCS -1/k f a Unitary regime -- crossover No phase transition through crossover

23 Vortices in trapped Fermi gases: marker of superfluidity M.W. Zwierlein, J.R. Abo-Shaeer, A. Schirotzek, C.H. Schunck, and W. Ketterle, Nature 435, 1047 (2005) 6 Li Resonance at 834G B<834G = BEC B>834G = BCS BEC BCS

24 Superfluidity and pairing for unbalanced systems Trapped atoms: change relative populations of two states by hand QGP: balance of strange (s) quarks to light (u,d) depends on ratio of strange quark mass m s to chemical potential μ (>0)

25 Experiments on 6 Li with imbalanced populations of two hyperfine states, 1i and 2i MIT: Zwierlein et al., Science 311, 492 (2006); Nature 442, 54 (2006); Y. Shin et al., arxiv: Rice: G.B. Partridge, W. Li, R.I. Kamar, Y.A. Liao, and R.G.. Hulet Science 311, 503 (2006)., Fill trap with n 1 1i atoms, and n 2 2i atoms, with n 1 > n 2. Study spatial distribution, and existence of superfluidity for varying n 1 :n 2.

26 Phase diagram of trapped imbalanced Fermi gases Y. Shin, C. H. Schunck, A. Schirotzek, & W.Ketterle, arxiv: G.B. Partridge, W. Li, R.I. Kamar, Y.A. Liao, and R.G.. Hulet, Science 311, 503 (2006). normal halo superfluid core In trap geometry Superfluid: : second order transition to normal phase with increasing radius with gapless superfluid near boundary Unstable => phase separation: : first order transition

27 Vortices in imbalanced paired fermions (MIT) BEC side All 1i 1i = 2i BCS side BEC No. of vortices vs. population imbalance

28 John Bardeen the Super Conductor Bob & Anne Schrieffer with his students, for his 60 th birthday, 1968.